This invention relates to an improved process for cooling hydrogen-bearing synthesis gas via heat exchange and specifically to a more efficient design of heat exchangers for hydrogen-bearing synthesis gas cooling.
Hydrogen-bearing synthesis gas streams are generally produced from steam/hydrocarbon reforming (often referred as steam/methane reforming), coal gasification and partial oxidation processes. See, e.g., U.S. Pat. No. 4,113,441 (steam reforming of hydrocarbons), U.S. Pat. No. 4,352,675 (partial oxidation of coal), U.S. Pat. No. 4,566,880 (partial oxidation of coal), U.S. Pat. No. 4,999,029 (partial oxidation of liquid and/or solid fuels), U.S. Pat. No. 5,856,585 (partial oxidation of natural gas), U.S. Pat. No. 6,730,285 (partial oxidation of hydrocarbon feed), and U.S. Pat. No. 6,749,829 (steam reforming of natural gas).
Traditionally, the hydrogen-bearing synthesis gas product streams obtained from these processes have been cooled in shell and tube heat exchangers. See, for example, Fix et al., U.S. Pat. No. 5,246,063, which discloses a heart exchanger for cooling synthesis gas generated from a coal-gasification plant. The heat exchanger contains a plurality of heat-exchange pipes which are surrounded by a jacket. The pipes communicate at one end with a gas-intake chamber, and at their other end with a gas-outtake chamber. Synthesis gas from a coal-gasification plant enters the gas-intake chamber, passes through the pipes, and then enters the gas-outtake chamber. While passing through the pipes, the synthesis gas is cooled by water introduced into the jacket. The water is vaporized into steam which is then removed from the jacket.
Koog et al., U.S. Pat. No. 4,377,132, discloses another type of shell and tube heat exchanger for cooling synthesis gas. This synthesis gas cooler has two concentric so-called “water walls” within an outer shell. The water walls are each formed from a plurality of parallel tubes joined together by connecting fins to form a gas tight wall. Water flows within the tubes and is vaporized into steam. The synthesis gas flows on the outside of the tubes, first axially and then through the annular region formed between the two concentric water walls.
Decke et al., U.S. Pat. No. 6,051,195, discloses a more complicated synthesis gas cooling system comprising a radiant synthesis gas cooler, and two convective synthesis gas coolers both which include a water-cooling structure to provide heat-exchange via cooling water flowing in counterflow.
Plate-fin heat exchangers and tube heat exchangers, spiral-wound or externally finned, have long been employed to recover process heat. These exchangers are often employed to heat or cool a low-density gas stream located on the external (often finned) side against a higher density stream with higher heat transfer coefficient within the plates or tubes. The extended surface of the finned exterior pass allows (1) greater heat transfer surface than a bare tube or plate and (2) provides greater heat transfer at a correspondingly lower pressure drop than would be experienced with bare tubes or plates.
Heat exchangers having more than one fluid circulating through separate tube passes are known. Published US Application No. 2005/0092472 (Lewis) discloses a plate fin and tube or finned tube type heat exchanger wherein a first working fluid is made to flow on the exterior of finned tubes, and two or more additional working fluids are made to flow in separate tube circuits within the heat exchanger. In an Example, U.S. '472 describes an embodiment wherein the first working fluid flows over the finned exterior side and three additional working fluids flow within separate tube circuits within the heat exchanger. The first working fluid is a mixture of N2 and H2O. The second working fluid is, for example, natural gas. The third working fluid is water, and the fourth working fluid is also water.
See also Misage et al. (U.S. Pat. No. 4,781,241) which describes a heat exchanger for use with a fuel cell power plant. In the exchanger, reformer effluent passes over the exterior of tubes. The latter provide for the circulation of three different fluids, i.e., water, steam, and hydrocarbon fuel to be preheated. See, also, U.S. Pat. No. 3,277,958 (Taylor et al.), U.S. Pat. No. 3,294,161 (Wood), U.S. Pat. No. 4,546,818 (Nussbaum), U.S. Pat. No. 4,344,482 (Dietzsch), and U.S. Pat. No. 5,419,392 (Maruyama).
The prior art process of cooling the hydrogen-bearing synthesis gas stream is heat exchange via separate, individual heat exchangers. In each of these separate heat exchangers, the synthesis gas is cooled to the desired outlet temperature by heat exchange with a single process stream, such as the feed hydrocarbon stream, boiler feed water, demineralized water, ambient air and/or cooling water. This practice of cooling hydrogen-bearing synthesis gas in one or more shell and tube heat exchangers, with each heat exchanger using a single cooling medium, is relatively inefficient. Recent changes in the cost of the feedstock material combined with ever increasing economic pressure have created a demand for a more efficient and less costly process and apparatus to accomplish synthesis gas production, including more efficient and less costly procedures for cooling synthesis gas by heat exchange.